Soybean is an important food source and has been traditionally used in the production of soy food products such as boiled bean, soy milk, tofu, ganmo (fried tofu fritter), deep-fried tofu, frozen and dried tofu, natto (fermented soybeans), tempeh, miso, and soy sauce. The protein thereof is edible protein of good quality and has a variety of physical properties including gel properties, emulsifiability, and water retention properties. Traditionally, soy protein has been isolated from soybeans, and utilization of the protein in the food products has been promoted. In addition, in recent years, novel soy foods such as fabricated (hybrid) food products containing soy protein as a major ingredient have been proposed and established.
In particular, due to recent rising health concerns, there are increased needs for soybean-based food products and it is demanded to create values such as portability, convenience, gel texture improvement for increased ease of eating and drinking, in addition to values such as palatability and nutritional wellness.
Soybean storage proteins can be recovered with relative ease as a precipitated fraction by isoelectric precipitation at about pH 4.5. This fraction is usually used, i.e., in the form of a “soybean protein isolate,” in the food manufacture industries. For example, a soybean protein isolate is used, with or without further processing, as an additive to other food materials for preparing daily dishes such as soy protein-based foods, meat products, and fish paste products, as well as favorite food such as snacks, nutritional bars, pastries and beverages; pure vegetable edible materials; and special purpose food products such as food preparations for patients allergic to milk or eggs and for patients with difficulty in chewing and/or swallowing, high protein content foods, and nutritionally balanced foods.
The soybean storage proteins are classified into 2S-, 7S-, 11S-, and 15S-globulins based on the sedimentation coefficients determined by ultracentrifugation analysis. Among them, 7S- and 11S-globulins are the main constituent protein components of the globulin fractions. On the other hand, Samoto et al. has reported that there is a group of proteins that have a high affinity for polar lipid which constitute membranes, including cell membranes, and protein body and oil body membranes among the proteins derived from soybeans (lipophilic protein), and that the lipophilic protein comprises as much as about 35% of an industrially produced soybean protein isolate (non-Patent Document 1).
The term “lipophilic protein” is a general term for a group of proteins that includes membrane proteins as major members of the group, particularly those with molecular weights of 34 kDa, 24 kDa and 18 kDa as estimated by SDS-polyacrylamide gel electrophoresis. These proteins are known to contain about 10 to 12% by weight of polar lipid which is extracted with a polar solvent mixture of chloroform:ethanol (2:1).
Thus, it has been revealed that lipophilic protein is included in the protein components of a widely-available soybean protein isolate, in addition to 7S- and 11S-globulins. It has become possible to fractionate these respective proteins from soybeans food-industrially (Patent Documents 1 and 2, and non-Patent Document 2).
However, the physical properties of the individual proteins, particularly those of the lipophilic protein, have not been fully elucidated.
On the other hand, methods are known for modifying edible proteins by cross-linking with transglutaminase. This enzyme catalyzes an acyl transfer reaction of the γ-carboxamide group of glutamine residue within a peptide chain. Transglutaminase forms a ε-(γ-Glu)-Lys cross-link within a protein molecule or between protein molecules by acting on the ε-amino group of lysine residue in the protein that acts as an acyl acceptor. When water acts as an acyl acceptor, deamidation reaction of a glutamine residue proceeds to form a glutamic acid residue. With respect to examples of the reaction of transglutaminase on soy protein, as shown in Patent Documents 3 to 5, gelling techniques comprising addition of transglutaminase to a soy protein-containing slurry have been disclosed.